A quadrature diode detector is commonly used in optical reader heads to sense the laser or light beam reflected from the storage medium. Amplified output signals from the diodes are used to control the focus servo system to keep the laser dynamically focused on the medium. Close control of the focus is necessary because the medium surface has large relative runout compared to the depth of field of the lens that focuses the light to a spot small enough to discern the one micron information bit size.
U.S. Pat. No. 4,368,526 shows a comparator for detecting the maximum peak of a differentiated read signal to establish the best focus in an open loop circuit. When the best open loop focus is established, the focus control loop is closed to operate as a servo loop using a quadrant diode detecting scheme.
U.S. Pat. No. 4,243,848 shows a focus controller using photodetectors disposed at one of the conjugate planes of the converging lens and others on the focal plane of the objective lens. The diodes are aligned in a direction corresponding to the movement of the images of the information bits as they are read. Using a phase relationship among the photodiode signals, a drive coil is controlled to maintain the light beam at best focus on the disk.
U.S Pat. No. 4,059,841, incorporated herein by reference, is directed to the use of four diodes arranged in quadrature to detect a projected quasipunctiform spot for providing feedback to a focus control circuit.
U.S. Pat. No. 4,163,149 is directed to the correction of a focus control signal error component caused by tracking offsets. It uses quadrature diodes to provide a focus control signal derived from the difference of opposed diode signals and a compensation signal from the sum of adjacent diode signals. That is, a correction signal is supplied to compensate for the lateral movement of the focus circle on the diodes due to the action of the tracking movement.
U.S. Pat. No. 4,190,775 discloses an optical memory playback apparatus using an electroluminescent semiconductor element as both the source of light and the sensor. The semiconductor element is a three-mirror resonator constructed of an outer mirror and a laser element. Information is stored in the form of variations in the reflection factor of the outer mirrors and is read out by allowing the variation in the light output, viz, the presence or absence of oscillation, directed onto the semiconductor element which also acts as the sensor.
None of the prior art shows, suggests, or teaches the use of a compensation to correct errors from misalignment, component aging, parameter changes, or such other causes of focus error. None of the prior art references teach, suggest, or show the use of the monitor diode output signal for detecting that the head is at best focus.